FIG. 1 shows a cross-sectional structure of a light emitting device employing a common organic electroluminescence element (organic EL element) and propagation of light. An electrode 102, a light emitting layer 103, and a transparent electrode 104 are stacked in this order on a substrate 101, and a transparent substrate 105 is provided on the transparent electrode 104. When a voltage is applied between the electrode 102 and the transparent electrode 104, light is radiated from a point S in the light emitting layer 103. The light enters the transparent electrode 104 directly or after being reflected by the electrode 102, and is then transmitted through the transparent electrode 104. The light transmitted through the transparent electrode 104 impinges on a surface of the transparent substrate 105 at point P at an incidence angle from the normal to the surface. At the point P, the light is refracted to be emitted into an air layer 106.
When the incidence angle θ exceeds the critical angle θc=sin−1 (1/n′1) where n′1 is the refractive index of the transparent substrate 105, total reflection occurs. For example, a light ray that is incident on the surface of the transparent substrate 105 at point Q at an angle greater than or equal to θc is totally reflected without being emitted into the air layer 106.
FIGS. 2(a) and 2(b) are diagrams for illustrating the light extraction efficiency of the light emitting device on the assumption that the transparent substrate 105 has a multilayer structure. In FIG. 2(a), the formula shown below holds according to Snell's law:n′k×sin θ′k=n′k-1×sin θ′k-1= . . . =n′1×sin θ′1=n0×sin θ0  (Formula 1)where n′k is the refractive index of the light emitting layer 103; n0 is the refractive index of the air layer 106; n′k-1, n′k-2, . . . , and n′1 are the refractive indices of a plurality of intervening transparent layers between the light emitting layer 103 and the air layer 106 in order of distance from the light emitting layer 103, closest first; θ′k is the propagation direction of a light ray radiated from the point S in the light emitting layer 3 (the angle from the normal to a refracting surface); and θ′k-1, θ′k-2, . . . , θ′1, and θ0 are the angles of refraction at the refracting surfaces in order of distance from the light emitting layer 103, closest first. Therefore, the formula shown below holds:sin θ′k=sin θ0×n0/n′k  (Formula 2)
Thus, Formula 2 is basically identical with Snell's law under the condition that the light emitting layer 103 is in direct contacts with the air layer 106. Formula 2 means that total reflection occurs when θ′k≧θc=sin−1(n0/n′k) irrespective of the refractive indices of the intervening transparent layers.
FIG. 2(b) schematically shows the range of light which can be extracted from the light emitting layer 103. The light which can be extracted is included in the extent of a pair of cones 107 and 107′ whose vertexes are at the light radiation point S. The vertex angle of each of the cones 107 and 107′ is twice the critical angle θc. The center axes of the cones 107 and 107′ are on the z-axis that is normal to the refracting surface. Assuming that the light is radiated from the point S with equal intensities in all directions and that the transmittance of light which is incident on the refracting surface at an incidence angle equal to or smaller than the critical angle is 100%, the extraction efficiency from the light emitting layer 103 is equal to the ratio of part of the surface area of the sphere 108 corresponding to the circular bases of the cones 107 and 107′ to the entire surface area of the sphere 108, and is expressed by the formula shown below:η=1−cos θc  (Formula 3)
Note that the actual extraction efficiency η is smaller than 1−cos θc because the transmittance for the incidence angles equal to or smaller than the critical angle does not reach 100%. The total efficiency of the light emitting element is equal to a value obtained by multiplying the above-described extraction efficiency η by the light emission efficiency of the light emitting layer.
Patent Document 1 discloses an organic EL element, in the context of the above-described mechanism, which is based on the concept that a diffraction grating is formed in a substrate interface, an internal surface, or a reflecting surface to change the incidence angle of light on a light extraction surface such that the light extraction efficiency is improved with the view of preventing total reflection of light propagating from the transparent substrate to the ambient air at the transparent substrate surface.
Patent Document 2 describes providing a plurality of protrusions over the surface of a transparent substrate of an organic EL element such that reflection of light at the interface between the transparent substrate and the air layer can be prevented, for the purpose of providing a planar light emitting device with excellent light extraction efficiency.